1. Field of the Invention
The present invention relates to a process for working up a liquid reaction discharge of the cationic polymerization of isobutene, which substantially comprises polyisobutene, unconverted isobutene and, if desired, an inert diluent.
2. Description of the Background
High molecular weight polyisobutenes having molecular weights up to several 100,000 Dalton have long been known and their preparation is described, for example, in H. Güterbock: Polyisobutylen und Mischpolymerisate, pages 77-104, Springer, Berlin 1959. The highly reactive polyisobutenes which have a high content of terminal vinylidene groups, preferably substantially more than 60 mol %, must be distinguished from these conventional polyisobutenes. Highly reactive polyisobutenes are desirable intermediates for the preparation of additives for lubricants and fuels.
Such highly reactive polyisobutenes are obtainable, for example, by the process of EP 0 628 575, by cationic polymerization of isobutene in the liquid phase with the aid of boron trifluoride and a secondary alcohol at from 0 to −60° C.
The prior patent applications DE 199 48 947.5, DE 199 52 031.3, DE 199 52 030.5, DE 100 28 585.6 and DE 100 35 298.7 relate to improved or advantageous embodiments of such a process.
After the desired molecular weight has been reached, the reaction is stopped by deactivating the polymerization catalyst. The polymerization catalyst or its deactivation products is or are preferably removed by extraction with an aqueous solution. The unconverted isobutene, possibly together with part of the inert diluent, is then distilled off, the polyisobutene formed and the main amount of any inert diluent remaining behind. However, this distillation has the considerable problem that a bulky stubborn foam forms. As a result of the foam formation, the column used for the separation may be flooded and may no longer be able to perform its separation task. The production has to be stopped or the production rate reduced until the foam collapses under the action of gravitational force. However, the total capacity of a production plant is thus greatly restricted.
The cause of the foam formation is not exactly known. Usually, the formation of foams is observed in the case of amphiphilic molecules. However, polyisobutene and any diluent used have only hydrophobic groups but not hydrophilic groups. The use of chemical antifoams is not suitable since the polyisobutene is obtained as a distillation residue and the added antifoam would remain in the end product.